Semiconductor laser device

ABSTRACT

A visible light semiconductor laser device has a double heterojunction structure comprising AlGaInP series semiconductors which is produced on a substrate having an electrode on its rear surface. A p type GaInP buffer layer is provided between a p type GaAs contact layer and a p type AlGaInP cladding layer. The buffer layer has a constant composition ratio and the energy band gap is smaller at the p type GaAs contact layer than at the p type AlGaInP cladding layer. Thus, an energy discontinuity between the p type GaAs contact layer and the p type AlGaInP cladding layer are relaxed for operation of the laser at lower voltage.

FIELD OF THE INVENTION

The present invention relates to a visible light semiconductor laser device having a double heretojunction structure comprising AlGaInP series semiconductors and a method for producing the same and, more particularly, to reducing laser operating voltage.

BACKGROUND OF THE INVENTION

FIG. 4 is a cross-sectional view showing a prior art semiconductor laser device. In FIG. 4, Reference numeral 1 designates an n type GaAs substrate. An n type Al₀.25 Ga₀.25 In₀.5 P cladding layer 2 is disposed on the substrate 1. An undoped Ga₀.5 In₀.5 P active layer 3 is disposed on the cladding layer 2. A p type Al₀.25 Ga₀.25 In₀.5 P cladding layer 4 is disposed on the active layer 3. An n type GaAs current blocking layer 5 is disposed on the cladding layer 4. A p type Ga₀.5 In₀.5 P buffer layer 6 is disposed on the cladding layer 4. A p type GaAs contact layer 8 is disposed on the current blocking layer 5 and the buffer layer 6. An n side electrode 10 is disposed on the rear surface of the substrate 1 and a p side electrode 11 is disposed on the contact layer 8. Reference numeral 9 designates a ridge approximately 3 to 6 microns in width. The distance between the current blocking layer 5 and the active layer 3 is approximately 0.3 microns. Respective layers 2 to 8 are crystal grown by metalorganic chemical vapor deposition (MOCVD).

The device will operate as follows.

When a forward direction voltage is applied between the p side electrode 11 and the n side electrode 10, a current concentratedly flows through the ridge portion 9 because the current blocking layers 5 are provided sandwiching the ridge portion 9. The n type Al₀.25 Ga₀.25 In₀.5 P cladding layer 2, the Ga₀.5 In₀.5 P active layer 3, and the p type Al₀.25 Ga₀.25 In₀.5 P cladding layer 4 constitute a double heterojunction structure, and laser light is generated in a region of the active layer 3 directly below the ridge portion 9. The p type Al₀.25 Ga₀.25 In₀.5 P (energy band gap of 2.2 eV)/p type GaAs (energy band gap of 1.42 eV) heterojunction has a large energy band discontinuity therebetween and a large potential barrier exists in the valence band, and therefore a it is difficult for a current to flow through the heterojunction. Therefore, a p type Ga₀.5 In₀.5 P buffer layer 6 having an energy band gap of about 1.87 eV is inserted between the p type Al₀.25 Ga₀.25 In₀.5 P cladding layer 4 and the p type GaAs contact layer 8 to relax the energy band discontinuity. If the buffer layer 6 is not provided, a predetermined laser light output power cannot be obtained unless the applied voltage is increased. The operating voltage can be reduced to a great extent by inserting the buffer layer 6.

The width of the ridge portion 9 mainly affects the transverse mode control. In this semiconductor laser device, since the distance between the active layer 3 and the current blocking layer 5 is about 0.3 micron and the thickness of the active layer 3 is below 0.1 micron, a portion of the light generated in the active layer 3 reaches the current blocking layer 5. However, since the energy band gap of the current blocking layer 5 is smaller than that of the active layer 3, a portion of the light generated in the active layer 3 is absorbed by the current blocking layer 5. On the other hand, no light absorption occurs in the ridge portion 9. Accordingly, a so-called "loss guide" (optical waveguide mechanism utilizing light absorption loss) is produced and a stable fundamental transverse mode is obtained by setting the ridge width at approximately 3 to 6 microns.

The p type Al₀.25 Ga₀.25 In₀.5 P cladding layer 4 has a large resistivity of approximately 0.6 to 0.8 Ωcm irrespective of doping and the ridge width thereof is approximately 3 to 6 microns so as to control the transverse mode. Therefore, this cladding layer 4 is likely to have a high resistance and increases the operating voltage. In order to reduce the operating voltage, it is necessary to relax the band edge discontinuity between p type GaAs and p type AlGaInP.

In the prior art semiconductor laser device constituted as described above, the p type Ga₀.5 In₀.5 P buffer layer 6 is inserted between the p type Al₀.25 Ga₀.25 In₀.5 P cladding layer 4 and the p type GaAs contact layer 8 to relax the band discontinuity between the p type AlGaInP and the p type GaAs, and to thereby reduce the operating voltage. However, even the buffer layer 6 is not sufficient.

SUMMARY OF THE INVENTION

The present invention is directed to solving the abovedescribed problems and has for its object to provide an AlGaInP series visible light semiconductor laser device relaxing the energy band discontinuity between the p type GaAs and the p type AlGaInP and operating at a low voltage.

Other object and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

According to a semiconductor laser device in accordance with the present invention, a p type GaInP buffer layer of a constant composition ratio is produced between a p type GaAs contact layer and a p type AlGaInP cladding layer. The energy band gap of this buffer layer is made smaller at the side of the p type GaAs contact layer than at the side of the p type AlGaInP cladding layer. Therefore, the band discontinuity between the p type GaAs and the p type AlGaInP can be relaxed sufficiently, whereby the operating voltage can be reduced.

According to a method for producing a semiconductor laser device in accordance with the present invention, while a p type GaInP buffer layer of a constant composition ratio is produced between a p type GaAs contact layer and a p type AlGaInP cladding layer, a portion of the buffer layer adjacent to the p type GaAs contact layer is grown at a temperature lower than the temperature for growing the portion of the buffer layer adjacent to the p type AlGaInP cladding layer. Therefore, a p type GaInP buffer layer whose energy band gap at the side of the p type GaAs contact layer is smaller than that at the side of the p type AlGaInP cladding layer can be easily produced. Furthermore, the p type carrier concentration of the buffer layer at the side of the p type GaAs contact layer is increased, thereby reducing the resistance.

According to a method for producing a semiconductor laser device in accordance with the present invention, while a p type GaInP buffer layer of a constant composition ratio is produced between a p type GaAs contact layer and a p type AlGaInP cladding layer, a portion of the buffer layer adjacent to the p type GaAs contact layer is grown in an atmosphere having a V/III (group five to group three) ratio larger than that ratio for growing another portion of the buffer layer adjacent to the p type AlGaInP cladding layer. Therefore, a p type GaInP buffer layer whose energy band gap is smaller at the side of the p type GaAs contact layer than at the side of the p type AlGaInP cladding layer can be easily produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an AlGaInP series visible light semiconductor laser device in accordance with a first embodiment of the present invention;

FIG. 2 is a diagram showing the relation between the energy band gap of Ga₀.5 In₀.5 P and crystal growth conditions;

FIG. 3 is a cross-sectional view showing an AlGaInP series visible light semiconductor laser device in accordance with a second embodiment of the present invention; and

FIG. 4 is a cross-sectional view showing an AlGaInP series visible light semiconductor laser device in accordance with the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a cross-sectional view showing an AlGaInP series visible light semiconductor laser device in accordance with an embodiment of the present invention. In FIG. 1, reference numeral 1 designates an n type GaAs substrate. An n type Al₀.25 Ga₀.25 In₀.5 P cladding layer 2 is disposed on the substrate 1. An undoped Ga₀.5 In₀.5 P active layer 3 is disposed on the cladding layer 2. A p type Al₀.25 Ga₀.25 In₀.5 P cladding layer 4 is disposed on the active layer 3. An n type GaAs current blocking layer 5 is disposed on the cladding layer 4. A p type Ga₀.5 In₀.5 P buffer layer 6 is disposed on the cladding layer 4. A p type Ga₀.5 In₀.5 P buffer layer 7 which is grown at a relatively low temperature is disposed on the buffer layer 6. A p type GaAs contact layer 8 is disposed on the current blocking layer 5 and the buffer layer 7. An n side electrode 10 is disposed on the rear surface of the substrate 1 and a p side electrode 11 is disposed on the contact layer 8. Reference numeral 9 designates a ridge portion. Here, the respective layers 2 to 6 are grown at 650° C. in an atmosphere having a V/III ratio of 400. The low temperature grown buffer layer 7 is grown while gradually decreasing the temperature from 650° C. to 600° C. The buffer layer 6 is about 0.05 micron thick and the low temperature grown buffer layer 7 is about 0.05 micron thick. MOCVD is employed as the crystal growth method.

Unlike AlGaAs series or InGaAsP series materials, Ga_(x) In_(1-x) P has specific characteristics. That is, Ga_(x) In_(1-x) P lattice matches with GaAs when x=0.5, but its energy band gap is not determined unconditionally when only the composition ratio is determined.

FIG. 2 shows the relation between the energy band gap of Ga₀.5 In₀.5 P which lattice matches with GaAs and the growth temperature and a relation between the energy band gap and the V/III ratio (mol ratio of V group element and III group element in a vapor phase) in a case where the crystal growth is performed by MOCVD.

When the growth temperature is high, the energy band gap is large, and when the growth temperature is low, the energy band gap is small. In this embodiment, utilizing this characteristic, the energy band gap of GaInP at the side of AlGaInP is made large and that at the side of GaAs is made small. Therefore, the energy band gap continuously varies and the energy band discontinuity is sufficiently relaxed, resulting in a visible light semiconductor laser having low operating voltage.

The device will operate as follows.

In the semiconductor laser device constituted as described above, the energy band gaps of the p type Al₀.25 Ga₀.25 In₀.5 P cladding layer 4, the p type Ga₀.5 In₀.5 P buffer layer 6, the low temperature grown p type Ga₀.5 In₀.5 P buffer layer 7, and the p type GaAs contact layer 8 gradually change as 2.2 eV, 1.87 eV, 1.42 eV, and the potential barrier at the side of the valence band is relaxed, resulting in a low operating voltage. Generation of laser light, transverse mode control and the like are the same as the conventional device. In addition, by decreasing the growth temperature of the buffer layer, the p type dopant (for example, zinc) is easily surrounded by the growth layers. Then, the carrier concentration increases, and the resistance thereof is reduced and the potential barrier is relaxed.

FIG. 3 shows a semiconductor laser device in accordance with another embodiment of the present invention. In FIG. 3, the same reference numerals designate the same or corresponding portions as those shown in FIG. 1. Reference numeral 12 designates a stripe-shaped groove produced in the current blocking layers 5, through which current flows confined by the current blocking layers 5. While in the laser device of FIG. 1 the p type Ga₀.5 In₀.5 P buffer layer 6 and the low temperature grown p type Ga₀.5 In₀.5 P buffer layer 7 are disposed on the ridge portion 9, in this embodiment they are disposed in the groove 12. Also in the structure of FIG. 3, in order to relax the energy band discontinuity between the p type Al₀.25 Ga₀.25 In₀.5 P cladding layer 4 and the p type GaAs contact layer 8, the p type Ga₀.5 In₀.5 P buffer layer 6 and the low temperature grown p type Ga₀.5 In₀.5 P buffer layer 7 function effectively.

In addition, although the energy band gap of the buffer layer is varied by changing the growth temperature in the above embodiments, the energy band gap can be varied by changing the V/III ratio as shown in FIG. 2. Therefore, while growing the p type Ga₀.5 In₀.5 P buffer layer, the V/III ratio of the buffer layer at the side of AlGaInP is lowered and that at the side of GaAs is increased. In this case, also the same effect as in the above-described embodiments can be obtained.

As is evident from the foregoing description, according to a semiconductor laser device of the present invention, a p type GaInP buffer layer of a constant composition ratio is produced between a p type GaAs contact layer and a p type AlGaInP cladding layer. The energy band gap of the p type GaInP buffer layer is made smaller at the p type GaAs contact layer than at the p type AlGaInP cladding layer. Therefore, an energy band discontinuity between the p type GaAs and the p type AlGaInP can be relaxed sufficiently, and a visible light semiconductor laser device having a low operating voltage can be obtained.

According to a method for producing a semiconductor laser device, when a p type GaInP buffer layer of a constant composition ratio is produced between a p type GaAs contact layer and a p type AlGaInP cladding layer, a portion of the buffer layer adjacent to the p type GaAs contact layer is grown at a temperature lower than a temperature for growing the portion of the buffer layer adjacent to the p type AlGaInP cladding layer. Or, a portion of the buffer layer adjacent to the p type GaAs contact layer is grown in an atmosphere having a V/III ratio larger than that ratio for growing the portion of the buffer layer adjacent to the p type AlGaInP cladding layer. Therefore, the p type GaInP buffer layer whose energy band gap is smaller at the p type GaAs contact layer than at the side of the p type AlGaInP cladding layer can be easily produced. Furthermore, in the method of varying the growth temperature, p type carrier concentration of the buffer layer at the side of the p type GaAs contact layer can be increased, resulting in a low resistivity. 

What is claimed is:
 1. A visible light semiconductor laser device having a double heterojunction structure comprising AlGaInP series semiconductors including:a first conductivity type semiconductor substrate having opposed front and rear surfaces; a first conductivity type AlGaInP first cladding layer, a GaInP active layer, and a second conductivity type AlGaInP second cladding layer successively disposed on the front surface of said semiconductor substrate; a second conductivity type GaInP buffer layer disposed on said second cladding layer and a second conductivity type GaAs contact layer disposed on said GaInP buffer layer, said GaInP buffer layer having a constant composition ratio and a smaller energy band gap at said GaAs contact layer than at said AlGaInP second cladding layer; a first electrode disposed on the rear surface of said substrate; and a second electrode disposed on said GaAs contact layer.
 2. A semiconductor laser as defined in claim 1 wherein said first and second cladding layers comprise Al₀.25 Ga₀.25 In₀.5 P and said buffer layer comprises Ga₀.5 In₀.5 P.
 3. A semiconductor laser device as defined in claim 2 wherein said buffer layer comprises a first GaInP layer disposed on said Al₀.25 Ga₀.25 In₀.5 P cladding layer and a second GaInP layer disposed on said first GaInP layer, said second GaInP layer having the same composition ratio as said first GaInP layer and an energy band gap smaller than said first GaInP layer, wherein said GaAs contact layer is disposed on said second GaInP layer.
 4. A semiconductor laser device as defined in claim 1 wherein said buffer layer comprises a first GaInP buffer layer disposed on said second cladding layer, and a second GaInP buffer layer disposed on said first buffer layer having the same composition ratio as said first buffer layer and an energy band gap smaller than said first buffer layer, said GaAs contact layer being disposed on said second buffer layer.
 5. A semiconductor laser device as defined in claim 1 wherein the first conductivity type is n type and the second conductivity type is p type.
 6. A visible light semiconductor laser comprising:a substrate; a first cladding layer disposed on the substrate; an active layer disposed on the first cladding layer; a second cladding layer disposed on the active layer; a Ga_(x) In_(1-x) P buffer layer having an energy band gap and disposed on the second cladding layer; a contact layer disposed on the buffer layer; and first and second electrodes disposed on the substrate and contact layers, respectively, wherein each of the layers is chosen from the group consisting of GaAs, AlGaInP, and GaInP, x is constant between the second cladding layer and contact layer, and the energy band gap of the buffer layer is smaller adjacent the contact layer than adjacent the second cladding layer. 